Читать книгу Physics of the Terrestrial Environment, Subtle Matter and Height of the Atmosphere - Eric Chassefiere - Страница 9

This book presents the methods developed in the 17th and 18th centuries for estimating the height of the atmosphere, methods essentially based on the observation of the light refracted and reflected by the atmosphere, and of certain meteors. As a result, these methods gave rise to a number of contradictions and advances. This question of determining the height of the atmosphere is inseparable from how the atmosphere is represented, with the atmosphere passing from the status of an idealized and purely mathematical object up to the middle of the 17th century, to that of a complex, eminently variable physical object, whose nature was the subject of a multitude of hypotheses, and which 18th-century scientists tried to reconcile within a coherent overall vision. The quest to determine how high the atmosphere reached, which the different phenomena studied did not all place at the same height, proved a powerful motivator of new research and debate during this time, as well as for the necessary syntheses that resulted. Rather than addressing, one by one, the different methods used to estimate the height of the atmosphere, in this book, we adopt a transverse approach by examining the various matters of a more subtle nature introduced by the scientists of the time to explain the phenomena from which they derived this height – the milestones in the construction of the representation of the atmosphere as a physical object. These subtle matters, most of which are part of the legacy of René Descartes, offer a broad field of investigation, which allows us to set the stage for the evolution of the scientific thinking of the time in relation to the understanding of atmospheric phenomena. There is a strong overall coherence between the hypotheses formulated at that time, far beyond the question of the validity of these hypotheses with respect to today’s knowledge, showing to what extent their abundance and the confrontation of points of view allow knowledge to mature, until the conceptual leap that overturned old ideas and allowed for an objective advance took place. These processes occurred over a considerable period of time, on the scale of over a century. It is this work of progressive maturation – the product of a permanent tension between the legacy of old ideas and the new ideas resulting from ever more numerous, rich and precise observations – which we present in this book.

Chapter 1 provides an analysis of the meaning of words used in the 18th century to characterize air, atmosphere, ether and, more generally, subtle matters. Understanding the literature of the time on the atmosphere and subtle fluids requires a good understanding of the exact meaning of the terms used, which often differs from the meaning given to them today. For example, the term vapor, which we apply today to a gas, was used at that time to refer to a component of small particles emanating from the Earth or water, the union of which can eventually lead to the formation of clouds or mists. The term air takes on different meanings depending on whether it is, for example, coarse air or elementary air. There are similarly varying definitions of ether, which surrounds and, according to some, penetrates the atmosphere, in the latter case providing it with such characteristics as, for example, its elasticity. The proliferation of scientific ideas in the 18th century is accompanied by a multiplicity of meanings given to different words, which is necessary to keep in mind to fully understand the nature of the mechanisms that are analyzed. Chapter 1, which is not intended to be an exhaustive study, provides illuminating observations on the basis of the analysis of a number of articles on different words concerning the atmosphere in several dictionaries: the Dictionnaire Universel de Furetière, whose first edition dates back to 1690; the Encyclopédie by Diderot and d’Alembert, published from 1751; and, as a comparison between the terms used by the French and English scientific communities, the Lexicon Technicum, whose first edition dates back to 1704. There are many parallels between the words used in France and across the channel in England, with dictionary articles from either country frequently quoting authors from the other country, but there are also differences, which are due to the scientific conceptions underlying the use of the words, which in the case of the French articles are infused with Cartesian doctrine. The use of the three dictionaries also makes it possible to note certain progressions that took place in the definition of scientific words between the dawn and the middle of the Enlightenment, in a period of rapid development of scientific thought.

Chapters 2–6 focus on five subtle matters of great importance to our subject, namely refractive matter (Chapter 2), proposed in particular by Jacques Cassini at the turn of the 18th century to remove the inconsistencies of the theory attributing to vapors and exhalations, a major role in refraction; solar matter (Chapter 3), which Jean-Jacques Dortous de Mairan used to explain the aurora borealis, based on the observations of zodiacal light made by Jean-Dominique Cassini a few decades earlier; magnetic matter (Chapter 4), initially proposed by Descartes on the basis of his theory of magnetism, and which Edmond Halley invoked to give his own explanation of the aurora borealis; electrical matter (Chapter 5), which was suggested to account for meteoroids, which ignite as they enter the atmosphere (“fiery meteors”, as we will call them in this book), and shooting stars (“falling stars”, as we will call them), objects that remained mysterious during most of the 18th century, and to which an electrical origin was attributed, as well as the aurora borealis after the theories of Mairan and Halley; and finally, subtle air (Chapter 6), invoked by Mairan in support of his theory of the aurora borealis to explain the suspension of solar matter at great heights.

Chapter 2 is devoted to refractive matter. After setting the context in terms of representations of the atmosphere at the end of the 17th century, following the discovery of its heavy nature and the elasticity of air, which made the atmosphere a physical object directly observable and measurable in a laboratory and in nature, we analyze the arguments used at the beginning of the 18th century in favor of the existence of a specific refractive matter that escapes the measurements of the barometer, in order to explain the observations of the refraction of starlight by the atmosphere. We show that this idea of a refractive matter fits well with the Cartesian thought dominant in the French Academy of Sciences at the time, and its contradictions, arising in particular from the inconsistency between the supposed major role, at the theoretical level, of condensed vapors in the process of atmospheric refraction, and the observation which, on the contrary, does not show a link between refraction and the presence of particles in suspension. This idea did not take hold or see any development across the Channel, where, at the end of the 17th century, Isaac Newton understood the essential role played by air temperature, and Halley the role of winds in the modulation of atmospheric pressure, without having to resort to the effect of vapors and exhalations. A beneficial side effect of the introduction of refractive matter has been the development of parametric models, using the differential approach, of refraction, such as that of Pierre Bouguer. These models, initially developed by French and English scientists, allowed for the creation of detailed models, including, at the middle of the 18th century, the precise consideration of temperature, and leading at the end of the century to the totally coherent model by Pierre Simon de Laplace, which signaled the definitive abandonment of refractive matter.

Chapter 3 deals with the solar atmosphere. We first examine the rich and abundant landscape at the end of the 17th century for the conceptions of sunspots, zodiacal light and comets, seen as phenomena in close relation to each other through an active solar atmosphere in many compartments of interplanetary space and planets. Then we consider the theory of the aurora borealis formulated by Jean-Jacques Dortous de Mairan at the beginning of the 18th century, namely the episodic precipitation in the Earth’s atmosphere of a subtle solar matter purported to mix with atmospheric matter and become luminous as a result of this mixture. We show how this theory fits into the framework of thought resulting from the previous two centuries, give the estimates of the height of the auroral structures made by Mairan and other scientists, put the theory in the context of the major currents of thought of the time, in particular the tendency of Cartesians to considerably increase the height of the atmosphere at the beginning of the 18th century, and detail the competing theories developed by Edmond Halley and Leonhard Euler. We also examine the impact of the existence of a solar atmosphere on the height of the atmosphere deduced from the duration of twilight periods, a question addressed at the beginning of the previous century by Johannes Kepler, who did not give the atmosphere a height greater than a few kilometers, and to which Philippe de La Hire, a century later, provides elements for an answer.

In Chapter 4, we examine the question of magnetic matter. At the beginning of the 18th century, Halley, witness to an aurora borealis, had the intuition that the luminous figures of the aurora are the visual manifestation of magnetic matter that circulates from one pole to the other in the upper atmosphere of the Earth, or the ether, following the Cartesian representation of the vortex of the magnet. This intuition is dictated to him by the disposition of the iron filings spread in the vicinity of a magnet, reminiscent of the auroral beams. This idea was taken up again by Charles François de Cisternay du Fay, studying a few years later the properties of magnets, as a proof of the circulation of the magnetic matter in only one direction, and not in both directions, as Descartes supposed in his system of the world. This question of the circulation of magnetic matter, and in particular that of its direction of flow in the magnet, which is at the heart of Halley’s system, preoccupied many scientists during this period, who carried out experiments to try to make magnetic matter apparent and to characterize it. Thus, the aurora borealis, as a life-size experiment revealing the Earth’s magnetic matter, took a central place in this period of progressive evolution in the understanding of the nature of the magnet, which led in the second half of the 18th century to the abandonment of the notion of the circulation of magnetic matter. In this chapter, we present the 17th-century context of Halley’s thought, the details of his explanation and the consequences of his work in the field of magnetism, as well as the more general evolution of the understanding of the magnet in his time and up to the end of the 18th century.

Chapter 5 concentrates on electrical matter. It clarifies the nature of “fiery meteors” (the bodies entering the atmosphere, or meteoroids in the current scientific terminology), whose first documented observations date back to the 17th century, and which provide information on the height of the atmosphere, extended over the entire 18th century. The extraterrestrial origin of these bodies, hypothesized as early as the end of the 17th century, but which violates the Cartesian doctrine of meteors inherited from Aristotle, took a very long time to gain acceptance, between the first scientifically substantiated proposal made by Halley at the beginning of the 18th century and the essentially accurate theory proposed by Ernst Chladni at the end of the same century. Few articles on the subject that mark out the 18th century show that, around the middle of the century, there was a period of rapid development in the experimentation on atmospheric electricity using instruments such as metal spikes and kites, which led to the discovery of the electrical nature of thunderstorms, the flow of electricity as an essential agent of fiery meteors and falling stars (i.e. shooting stars), as well as the aurora borealis, with these different phenomena being considered by many scientists of the time as closely related. In the second half of the 18th century, there was a general tendency to attribute many phenomena involving fire to electricity, and the meteors mentioned above are no exception to this rule. The electrical nature of fiery meteors was hotly debated during this period, the electrical hypothesis having its fierce supporters and detractors, who expressed doubts on the basis of observations, some aspects of which they considered contradictory with the electrical nature of the phenomenon.

Chapter 6 addresses the question of subtle air, more delicate than the air we breathe, but less subtle than ether, in relation to the aurora borealis theory of Mairan. The extremely tall estimate of the height of the auroral structures, which rotate with the Earth and were assumed by most scientists of the time to be immersed in its upper atmosphere, suggests that the auroral matter, which Mairan supposed to come from the solar atmosphere, mixes with very fine air, extending much higher than the coarse air, whose pressure is given to us by the mercury barometer. In this chapter, we analyze the conceptual framework from the 17th century in which Mairan’s system must be placed, as well as the experimental evidence on which he and his contemporaries relied to postulate the existence of subtle air: unequal mercury levels in different barometers, suspension of mercury at great heights in inverted tubes, “mercurial phosphorus” (the luminous barometers), considerable degree of adhesion between joined polished planes and theories of the coherence of solid bodies. We show how the subtle air hypothesis allowed Mairan to overcome difficulties resulting from divergent estimates of the height of the atmosphere using different methods, and how, thanks to the introduction of subtle air, a coherent picture of the vertical structure of the atmosphere emerged from the work carried out in the second half of the 17th century and throughout the 18th century.

Chapter 7 is devoted to a synthesis of the estimates of the height of the atmosphere made from the various methods used, the interpretation of which involves one or more of the subtle matters analyzed in the previous chapters: atmospheric refraction (refractive matter), the duration of twilight periods (solar matter), the aurora borealis (solar and magnetic matters, subtle air), fiery meteors, and falling stars (electrical matter). We describe the context, in terms of the representations of the atmosphere and the interpretation of the different phenomena used to estimate its height. We analyze, in the light of the then-dominant representations of the atmosphere, in particular the presence of a component considered to be of major importance, in terms of weight and refractive power, of vapors and exhalations, or the existence of a finer, even subtle component, of the atmosphere extending at a very great height above the Earth, and of the conceptions developed at the turn of the century on subtle matters, the contradictions that affected the evolution of how the height of the atmosphere was estimated by the various different methods, and the arguments developed to solve them. More generally, the introduction of these many subtle matters gave rise to very lively debates, which at the dawn of the 19th century led to a relatively unified, scientifically supported vision of the Earth’s atmosphere and its vertical extension. The numerous concepts developed at the time, following their own logic, within a different frame of thought from that of today, were translated into advances and tested by the observation of the atmosphere, constantly tending towards the search for an overall coherence of the representation of the physical object that, for the scientists of the Enlightenment, made up the atmosphere that surrounds us. In particular, this included the idea of a vertical stratification of the atmosphere, no longer based on Aristotelian categorizations, but on criteria of a physical nature relating to different characteristics such as density or the electrical state, for example.

In Chapter 8, we look at the question of the atmospheres of terrestrial bodies, which are at the juncture point between the atmosphere and subtle matter. We explore the vast field of the various exchange processes envisaged at that time between solid bodies and the Earth’s atmosphere, which are responsible for the particular atmospheres surrounding these bodies. These processes presuppose a circulation of matter through the pores of the bodies; large pores for thick vapors and air, and small pores for subtle matter. The porosity of bodies, in nature and in terms of physical characteristics, is an essential aspect of this question, and we first present the various observations and theories of the time on this subject. Then, we move on to an examination of the atmospheres of the bodies themselves, again detailing different observations and theories, and reviewing the various materials involved in the formation of these atmospheres: air, various vapors, electrical and magnetic fluids, igneous or caloric fluids, or luminous fluids, etc. These theories are in some cases based on particular representations of the atmosphere and the ether, and on principles of physics such as dissolution, applied to the ether or to the igneous fluid as solvents, and to earthly or aqueous bodies as dissolved substances within an air forming the mixture, which we try to put into perspective in the context of the time. Out of the abundance of ideas in this field, we see emerging a closely interconnected world, where solid bodies, their particular atmospheres and the Earth’s global atmosphere interact permanently through the circulation of subtle, or thick, materials within solid bodies and their atmospheres, through which they are in contact with each other, exchanging matter and movement.

In order to provide clarity on both the vocabulary and the scientific ideas of the time, we felt it necessary to illustrate our subject with numerous excerpts from texts. These excerpts, concerning sources in French, are translated into English. Concerning the sources in English, we have endeavored to provide the original source text.